Where it comes to faith, there's one role that reason plays, as Vatican I taught, and that is whether or not we find the AUTHORITY behind Catholic teaching to be credible; these are known as the "motives of credibility". We've decided that this Conciliar Church lacks the marks of the True Church, and therefore we do not submit to its authority. Once you've decided that this Church speaks for Christ, then you have to submit and give internal assent. Now, that is the same as absolute assent, in that it's possible for the pope to err here or there, but not substantially. To posit a Magisterium, as R&R do, that is thoroughly corrupt and harmful, and a Public Worship of the Church that is offensive to God, that's a blasphemy against the Holy Catholic Church and the Holy Spirit, Who guides her.
THIS is all sedevacantism is rooted in:
Msgr. Fenton:
To the Holy Father’s responsibility of caring for the sheep of Christ’s fold, there corresponds, on the part of the Church’s membership, the basic obligation of following his directions, in doctrinal as well as disciplinary matters. In this field, God has given the Holy Father a kind of infallibility distinct from the charism of doctrinal infallibility in the strict sense. He has so constructed and ordered the Church that those who follow the directives given to the entire kingdom of God on earth will never be brought into the position of ruining themselves spiritually through this obedience. Our Lord dwells within His Church in such a way that those who obey disciplinary and doctrinal directives of this society can never find themselves displeasing God through their adherence to the teachings and the commands given to the universal Church militant. Hence there can be no valid reason to discountenance even the non-infallible teaching authority of Christ’s vicar on earth.
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It is, of course, possible that the Church might come to modify its stand on some detail of teaching presented as non-infallible matter in a papal encyclical. The nature of the auctoritas providentiae doctrinalis within the Church is such, however, that this fallibility extends to questions of relatively minute detail or of particular application. The body of doctrine on the rights and duties of labor, on the Church and State, or on any other subject treated extensively in a series of papal letters directed to and normative for the entire Church militant could not be radically or completely erroneous. The infallible security Christ wills that His disciples should enjoy within His Church is utterly incompatible with such a possibility.